Genetic differences are sought by electrophoretic techniques on enzymes or proteins involved in the metabolism of neurotransmitters. 5-HTP Decarboxylase has been found to show differences in rat, mouse, beef, man. Rats have multiple electrophoretic forms, probably polymers, which are disaggregated by urea or triton. They seem to form polymers also when mixed, for example, with the mouse or beef enzyme which alone normally form only one molecular species. No difference in electrophoretic mobility was found between five mouse strains tested. These investigations are being extended to other enzymes and species.